Liquid crystal display elements are advantaged over other display elements in terms of its thin thickness, light weight, and low power consumption. The liquid crystal display elements are widely used in image display apparatuses such as televisions, video cassette recorders, and the like, and OA (Office Automation) apparatuses such as monitors, word processors, personal computers, and the like.
Conventionally known liquid crystal display methods of the liquid crystal display elements are, for example, the TN (Twisted Nematic) mode in which a nematic liquid crystal is used, display modes in which FLC (Ferroelectric Liquid crystal) or AFLC (Antiferroelectric Liquid crystal) is used, a polymer dispersion type liquid crystal display mode, and the like mode.
Among the liquid crystal display methods, for example, the TN (Twisted Nematic) mode in which the nematic liquid crystal is used is conventionally adopted in the liquid crystal display elements in practical use. The liquid crystal display elements using the TN mode have disadvantages of slow response, narrow viewing angle, and the like drawbacks. Those disadvantages are large hindrances for the TN mode to take over CRT (Cathode Ray Tube).
Moreover, the display modes in which the FLC or AFLC is used, are advantageous in their fast response and wide viewing angles, but significantly poor in anti-shock property and temperature characteristics. Therefore, the display modes in which the FLC or AFLC is used, have not been widely used practically.
Further, the polymer dispersion type liquid crystal display mode, which utilizes scattering of light, does not need polarizer and is capable of performing highly bright display. However, in principle, the polymer dispersion type liquid crystal display mode cannot control the viewing angle by using a phase plate (retardation film). Further, the polymer dispersion type liquid crystal display mode has a problem in terms of the response property. Thus, the polymer dispersion type liquid crystal display mode is not so advantageous over the TN mode.
In all those display methods, liquid crystal molecules are oriented in a certain direction and thus a displayed image looks differently depending on an angle between a line of vision and the liquid crystal molecules. On this account, all those display methods have viewing angle limits. Moreover, all the display methods utilize rotation of the liquid crystal molecules, the rotation caused by application of an electric field on the liquid crystal molecules. Because the liquid crystal molecules are rotated in alignment all together, responses take time in all the display method. The display modes in which the FLC and the AFLC are used, are advantageous in the response speed and the viewing angle, but have such a problem that their alignment would be irreversibly destroyed by an external force.
On contrary to those display methods in which the rotation of the molecules by the application of the electric field is utilized, a display method in which the secondary electro-optical effect is utilized.
The electro-optical effect is a phenomenon in which a refractive index of a material is changed by an external electric field. There are two types of the electro-optical effect: one is an effect proportional to the electric field and the other is proportional to the square of the electric field. The former is called the Pockels effect and the latter is called the Kerr effect. Especially the Kerr effect has been adopted in high-speed optical shutters early on, and has been practically used in a special measurement instruments. The Kerr effect was discovered by J. Kerr in 1875. So far, organic liquid such as nitrobenzene, carbon disulfide, and the like, are known as material showing the Kerr effect. Those materials are used, for example, in the aforementioned optical shutters, and the like devices. Further, those materials are used for measurement of strength of high electric fields for power cables and the like, and the like usage.
Later on, it was found that liquid crystal materials have a large Kerr constant. Researches on basic technology have been conducted to utilize the large Kerr constant of the liquid crystal materials for use in light modulation devices, light deflection devices, and further optical integrated circuit. It was reported that some liquid crystal compounds have a Kerr constant more than 200 times higher than that of nitrobenzene.
Under those circumstances, studies for utilization of the Kerr effect in display apparatuses has been started. It is expected that the utilization of the Kerr effect attains relatively a low voltage driving because the Kerr effect is proportional to the square of the electric field. Further, it is expected that the utilization of the Kerr effect attains a high-response display apparatus because the Kerr effect shows a response property of several μ seconds to several m seconds, as its basic nature.
Incidentally, one of large practical problems to be overcome for utilizing the Kerr effect in display elements is that the utilization of the Kerr effect requires a larger driving voltage compared with the conventional liquid crystal display elements. To solve this problem, Publication of Japanese Patent Application, publication No. 2001-249363 (Tokukai 2001-249363; published on Sep. 14, 2001) (hereinafter, referred to as Patent publication 1) teaches an art in which a display element in which orientation of negative type liquid crystalline molecules is carried out is provided with substrates having a surface that have been subjected to alignment treatment, in order that the Kerr effect may be easily generated in the display element.
In the display element described in Patent Document 1, the negative type liquid crystalline molecules are provided between a pair of substrates. Here, the wording “negative type” indicates that the liquid crystalline molecules shows negative dielectric anisotropy. Moreover, electrodes are provided respectively on inner sides of the substrates. Alignment films which have been treated with rubbing process are provided on the electrodes. Moreover, on outers sides of the substrates, polarizers are so provided that their absorption axes cross each other perpendicularly. Moreover, rubbing directions of the alignment films provided on the electrodes are parallel and directed in the same or opposite directions. Further, the rubbing directions make 45 degrees with the absorption axes of the polarizers.
In the display element of Patent Document 1 having the above arrangement, an electric field (voltage) is applied between the electrodes so as to generate the electric field along a normal direction of the substrates. When the electric field is applied, the polarized negative type liquid crystalline molecules are oriented along an electric field direction in such a manner that the molecules are so directed that their major axial directions is parallel to the rubbing direction (the electric field direction is a direction in which an electric field is applied). With this arrangement, the display element of Patent Document 1 attain optical response property in which its transmittance is increased by the electric field application.
The display element disclosed in the publication 1 has such a problem that when no electric field is applied, light leakage occurs thereby causing poor contrast, whereas when the electric field is applied, coloring phenomenon is caused.